The present invention relates generally to automated optical inspection (AOI) systems and particularly to an AOI system for inspecting and detecting defects in laser-drilled vias in printed electrical circuits.
Most multi-layer printed circuit boards (xe2x80x9cPCBxe2x80x9ds) have vias, i.e., passageways from one layer to another, which generally are plated to provide electrical contact between the layers. Vias may be formed in many ways, for example by means of apertures in the required x-y positions in each layer, such that when the layers are joined together one on top of another and properly registered, the apertures define the via path.
Recently vias have been made by first forming a laminated multi-layer board, and then by drilling the vias by means of photoablating and/or photo-cutting with a laser beam. Two examples of widely used lasers are frequency tripled (or quadrupled) pulsed YAG lasers (wavelength 355 or 266 nm) and CO2 lasers (wavelength about 10.6 xcexcm). Both types of lasers can easily cut PCB laminate materials, such as glass-epoxy or polyimide. These two types of lasers have different advantages and disadvantages. For example, the laser beam of a YAG laser cuts through copper. This characteristic offers the advantage of being able to use a YAG laser to prepare vias on substrates that are coated with copper, however one can overdrill the laminate (insulating layer) into the copper of the lower layer if precautions are not taken. Conversely, the laser beam of a CO2 laser does not cut through the copper, thus there is no danger of overdrilling. However a CO2 laser can not be used to prepare vias on substrates coated with copper. Consequently, when a CO2 laser is used, metal must be removed from the top layer where it is desired to drill a via, for example by etching, which does not have to be done for a YAG laser.
FIGS. 1A-1F illustrate different typical defects associated with laser-drilled vias. The figures show an upper copper layer 2, a laminate layer 3 and a lower copper pad 4, wherein it is desired to drill a via 5 from upper layer 2 down to the top of pad 4. FIG. 1A illustrates an underdrilled via, which of course means that although the via is plated with copper, no electrical connection will be made between layer 2 and pad 4. FIG. 1B illustrates an overdrilled via, which means that the via is drilled through pad 4 resulting in too little metal being left on pad 4 for a reliable electrical connection between layer 2 and pad 4. FIG. 1C illustrates xe2x80x9cthrow outxe2x80x9d, i.e., debris 6 from photoablation of the layers being left in via 5, which can cause plating or electrical connection problems. FIG. 1D illustrates the presence of foreign materials 7 in the via, which can cause plating or electrical connection problems. FIG. 1E illustrates an underplated via, namely a properly drilled via having plating 8 that is not suitably deposited which can lead to an insufficient electrical connection. FIG. 1F illustrates via 5 misregistered with pad 4, which, although not being a defect in the drilling process per se, nevertheless is a defect which must be detected because it too can lead to an insufficient electrical connection.
Although automated optical inspection (AOI) systems are typically used to inspect PCBs, nevertheless no AOI system is known which can accurately, repeatably and reliably detect various defects in laser-drilled vias, independently of whether or not the PCB under inspection, and the vias thereon, are cleaned prior to inspection. Moreover no laser drill repair station is known to inspect substantially all laser drill vias on a PCB and to automatically repair only those defective laser drill vias having defect types that are repairable.
U.S. Pat. No. 5,216,479 shows and describes an optical inspection system for inspecting a surface of a laminate operative to distinguish between a first material, such as a printed circuit board laminate, and a second material, such as copper formed thereon, and employing a laser illuminator selectively illuminating the surface and signal analyzers operative to sense and analyze fluorescent light and reflected light resulting from illumination by the laser.
The present invention seeks to provide a novel AOI system which can accurately, repeatably and reliably detect and distinguish various different defects in laser-drilled vias. The present invention can simultaneously inspect through holes and laser-drilled vias. Preferably the detection of the various different defects can be made independently of whether or not the PCB under inspection, and the vias thereon, are cleaned prior to inspection.
Additionally, the present invention seeks to provide a novel AOI system which can accurately, repeatably and reliably detect and distinguish various different defects in laser-drilled vias, and which can automatically repair, for example by re-drilling, those defective vias which are repairable. Preferably, if non-repairable defects are detected then no vias, such as vias having repairable defects, are redrilled.
The present invention uses a combination of inputs from at least two optical data channels which sense one or more of three different parameters of a radiation beam impinging on substances found on the PCB: luminescence of the substance (laminate, copper layer, copper pad, debris, etc.) due to the beam impinging thereon, reflection of the beam from the substance, and transmission of the beam at the point of impingement. Each data channel preferably is adjustable independently of the other channels for optimal performance. Although most preferably the beam is a laser beam, nevertheless the invention can be carried out with any coherent or non-coherent monochromatic or polychromatic light beam, or any other radiation, electromagnetic wave or acoustic beam, for example.
It is noted that the term xe2x80x9cluminescencexe2x80x9d refers to the emission of visible or non-visible electromagnetic radiation as a result of absorption of exciting energy in the form of photons, charged particles, or chemical change. The term luminescence includes both fluorescence and phosphorescence. In xe2x80x9cfluorescencexe2x80x9d, an atom or molecule emits detectable radiation in passing from a higher to a lower electron excitation state. The term fluorescence relates to phenomena in which the time interval between absorption and emission of energy is extremely short, typically in the range of 0.01-1000 microseconds. The term xe2x80x9cphosphorescencexe2x80x9d relates to the emission of radiation continuing after excitation has ceased, and may last from a fraction of a second to an hour or more.
The fluorescence of metallic conductors on PCBs, such as copper, under short wavelength visible light (for example 442 nm laser light emitted by a helium cadmium CW laser) is measurably less, generally by an order of magnitude, than the fluorescence of typical PCB laminate materials, such as glass-epoxy or polyamide. In addition, copper reflects light much better than such typical PCB laminate materials. The present invention exploits the large differences in fluorescence and reflectance of laminate compared to conductor in order to distinguish between portions of the via which are laminate and portions which are copper.
The reflectance and luminescence sensors preferably are placed above a horizontal PCB to be inspected, and are angled with respect to the via. The angled orientation of the sensors permits them to view and to provide sensed information along the entire depth of the via. By simultaneously combining and analyzing sensor inputs from at least two channels, the AOI system can detect and distinguish between at some of the various defects shown in FIGS. 1A-1F. The combined information can distinguish between defects in the via as opposed to defects on the upper surface of the PCB or at the bottom of the via. Most importantly, the combined information can distinguish between underdrilling or residue debris which can be repaired by further laser drilling, and other defects such as via hole misalignment that generally can not be readily repaired. Preferably the sensor inputs are signals from the combination of two of the luminescent, reflective and transmissive sensors respectively. Alternatively, the sensor inputs may be two signal inputs from the luminescent sensor, or two inputs from the reflective sensor, wherein each input is interpreted with reference to one of various thresholds of luminescence or reflectance, and then combined to detect the presence of defects.
The transmission sensor, simultaneously with the reflectance and luminescence sensors, can sense light that passes through the hole from one side of the PCB to another, thereby providing information regarding the position, and possible defects, of through holes. The sensitivity of the system can be adjusted as desired, in order to distinguish between defects which are sufficiently small in size so as not to pose any problem in plating or electrical conductivity, as opposed to those defects which are of a size sufficient to pose a problem. For example, by adjusting the sensitivity of the system, the system can distinguish between debris remaining in a via which may hinder plating or electrical conductivity, for example debris which is 3-4 xcexcm size, as opposed to debris which is not considered a hindrance to plating or electrical conductivity, for example debris of only 1 xcexcm size.
The system can also be used to scan a PCB before or after cleaning the PCB with a cleaning agent, or before or after plating. Preferably, when the system is used to inspect a PCB prior to cleaning, only one input is used, for example from the luminescence sensor, however the signal from the one sensor is analyzed with respect to two different thresholds and then combined to determine the presence of defects.
Additionally, the system can be used to inspect substantially all the vias on a region of a PCB and automatically distinguish between repairable and not repairable vias in the inspected region. Those vias which are deemed repairable are subsequently repaired. Preferably, repairable and not repairable vias are distinguished as a function of one or more of the type of defect and the location of the defective via. Thus, for example, underdrilled vias may be deemed repairable and thus may be subsequently automatically further drilled so that adequate electrical contact can be made with the affected via. Preferably, the system includes an intelligent decision tree so that if a PCB includes a non-repairable via, for example an overdrilled or misaligned via, none of the repairable vias on the PCB are automatically repaired in order to conserve repair resources.
There is thus provided in accordance with a preferred embodiment of the present invention an automated optical inspection system including an automated optical inspection system including a source of electromagnetic radiation for delivering a radiation beam on an article to be inspected;
a plurality of sensors arranged with respect to the radiation beam for sensing a plurality of radiation properties associated with the radiation beam impinging at least at a zone of impingement on a substance found on the article to be inspected, wherein the plurality of sensors includes a luminescence sensor for sensing luminescence of the substance due to the beam impinging thereon and a reflectance sensor for sensing reflectance of the beam from the substance and wherein the sensors transmit information signals based on the radiation properties sensed by said sensors; and
a processor in communication with said sensors operative to receive the information signals for a plurality of zones of impingement, to combine said signals from the sensors and to analyze them, and to generate an output indicating the presence of defects based on said analysis.
Preferably, a preferred embodiment of the system includes at least one or more of the following:
The defects include defects that are automatically repairable and defects that are not automatically repairable.
The luminescence sensor is a fluorescence sensor.
The article to be inspected is PCB with a via formed which has a depth, and the reflectance and luminescence sensors are positioned at an angle with respect to the via, such that said sensors can view and provide sensed information generally along the entire depth of said via.
The sensors have an adjustable sensitivity.
The sensitivity of each sensor is adjustable independently of the other.
The position of each sensor is adjustable independently of the other.
The radiation beam is a laser beam.
The processor comprises a filter to filter out a level of luminescence which could cause a false alarm.
The processor includes a filter to filter out a level of reflectance which could cause a false alarm.
The processor processes the information signals into binary signals with reference to predetermined detection thresholds for luminescence and reflectance.
The processor generates a binary image for each of luminescence and reflectance signals.
The processor multiplies the binary images together, and calculates a composite luminescence and reflectance image.
The processor compares the composite images to predetermined defect parameters, and produces a defect report.
The system includes a drilling laser unit operative to drill a hole or via in a PCB; the automated optical inspection system and said drilling laser are in electrical communication with a controller; and a position of the hole or via is fed from the automated optical inspection system to the controller, and the controller instructs the drilling laser to drill the hole or via.
The position of the hole or via is fed from the automated optical inspection unit if said hole or via is analyzed by the processor to be defective and repairable.
The drilling laser is operative to avoid drilling any holes or vias on said PCB if any said hole or via is determined by the automated optical inspection system to be defective and not repairable.
The drilling laser comprises a CO2 laser.
The controller is also the processor.
The drilling laser is said source of electromagnetic radiation for delivering a radiation beam on an article to be inspected.
There is thus provided in accordance with a preferred embodiment of the present invention an automated optical inspection system including:
a source of electromagnetic radiation for delivering a radiation beam onto an article to be inspected;
a sensor arranged with respect to the radiation beam for sensing a radiation property associated with said radiation beam impinging at least at a zone of impingement on a substance found on the article to be inspected, wherein the sensor transmits information signals based on the radiation properties sensed by said sensor; and
a processor in communication with said sensor, the processor being operative to:
receive information signals for a plurality of zones of impingement,
extract from the information signal additional information about said article by analysis of the information signals with reference to at least two different thresholds; and
generate an output indicating the presence of a defects based on analysis of the extracted information.
Preferred embodiments of the invention include one or more of the following:
The processor further includes an information combiner operative to combine additional information about said article obtained by analysis of the information signals with reference to the at least two different thresholds.
The detected defects include defects that are classified as automatically repairable and defects that classified as being not automatically repairable.
The sensor is a luminescence sensor operative to sense fluorescence.
The article to be inspected is a PCB with a via formed therein, wherein the via has a depth, and wherein the luminescence sensor is positioned at an angle with respect to the via, such that it can view and provide sensed luminescence information generally along the entire depth of said via.
The sensor has an adjustable sensitivity.
The radiation beam is a laser beam.
The processor includes a filter to filter out a level of luminescence which could cause a false alarm.
The processor processes the information signals to produce a set of binary images with reference to at least two different predetermined detection thresholds for luminescence.
The processor multiplies the binary images together, and calculates therefrom a composite image.
The processor compares the composite image to predetermined defect parameters, and produces a defect report based on the comparison.
The system further includes a drilling laser operative to drill a hole or via in a PCB; automated optical inspection system and the drilling laser are in electrical communication with a controller, and a position of the hole or via is fed from said automated optical inspection system to the controller, which instructs said drilling laser to drill the hole or via.
The position of the hole or via is fed from said automated optical inspection device if said hole or via is analyzed by the processor to be defective and repairable.
The drilling laser is operative to avoid drilling holes or vias on said PCB if upon inspection a hole or via on the PCB is determined by the automated optical inspection system to be defective and not repairable.
The laser comprises a CO2 laser.
The controller is said processor.
The drilling laser is the source of electromagnetic radiation for delivering a radiation beam on an article to be inspected.
There is thus provided in accordance with a preferred embodiment of the present invention, a system for repairing defective laser drilled holes in an electrical circuit to be inspected including:
an automated optical inspection subsystem operative to inspect a PCB and to provide output indications of defective and non-defective vias on the PCB, and
a controller in communication with the automated optical inspection system and with a laser drill, wherein the controller is operative to instruct the laser drill to automatically redrill at least some holes on the PCB indicated as being defective.
The output indication of repairable and non-repairable holes further indicates which defective holes are repairable, and the laser drill automatically redrills defective and repairable holes.
The laser drill avoids drilling any holes if the automated optical inspection system detects a defective and non-repairable hole.
The laser drill is a CO2 laser drill.
The laser is a YAG laser drill, and the automated optical inspection system determines a characteristic of repairable defective holes.
The characteristic of a repairable defective hole is the size of an artifact in the hole.
The characteristic of a repairable defective hole is the thickness of a residue laminate in the hole.
The laser drill is operative to provide an amount of energy adapted to clear the hole without overdrilling the hole.